[roll] Roll fuchsia [display][banjo] Remove OnDisplaysChanged() return value.

The return value of OnDisplaysChanged() does not suit us for a couple of
reasons.

1. The equivalent FIDL API will use an event, which cannot return
   information.

2. In some cases, populating the return value requires communicating
   with the display driver, and may take a long time. This is currently
   true when the coordinator has to read and parse the display's EDID.
   EDID displays are not an edge case -- in future designs, the
   coordinator may consult an on-disk or online database for
   model-specific configuration and workarounds.

Fortunately, the return value of OnDisplaysChanged() is not critical to
any of our current drivers. Taking advantage of our current situation,
this CL removes the return value, which is the fastest way to unblock
FIDL migration work. Future CLs will bring back passing display
information from the coordinator to the display drivers.

Original-Bug: b/314126995
Test: Booted core.x64 on NUC11
Test: Booted core.vim3 on VIM3 with HDMI monitor
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/965392
Original-Revision: 7864a186059d7a65f1931ce8384300b3dbc4279e
GitOrigin-RevId: aefd233f1c1201b0e439be4a3aa346843b4df4b2
Change-Id: I9148ffee4ca4674e5b5920f6e44564cad2103461